Error message on PayPal payment link for sold out event

I am using EE4 for payment for seminars. All registrations are set to be Approved automatically, as payment is not required at the time of booking, but people’s places are held for a week, when payment is due. As I don’t want an event to be overbooked (and I have a counter on the booking form which counts down the number of places available as bookings are made), I have set it up in this way. You can see an example of this form here: http://opendialogueapproach.co.uk/events/an-introduction-to-the-open-dialogue-approach-18/

I currently have an event which is sold out and for which payments still need to be made. Users are however seeing the following error message when they try to make a payment by PayPal: